When you store your pictures in your Google account, you have different options to upload them.

Google Photos:
whether you upload from your phone, using the "Desktop uploader"
(Google Photos Backup) or to the website Google Photos
(photos.google.com), you have these two options to choose from in the Google Photos settings:

"High quality" (free unlimited storage): this will

Resample your images to 16 MPixel (if larger)

Apply
compression on your pictures to reduce the storage needed for them on
the Google servers. More info about this can be found here: http://goo.gl/ZPDiVP

"Original": this will upload your images at full resolution and will ALWAYS consume storage even if they are <= 16 MPixel.

"Best
for web sharing (2048px)": Images will be resampled to 2048 pixel on
the longest edge and will be considered as free unlimited storage.

"Original size (slowest upload)": Images will upload at original size, but regardless of the resolution they will ALWAYS consume storage.

Important: After uploading in "Original size (slowest upload)" it is possible to "RECOVER STORAGE" in your Google Photos settings: https://photos.google.com/settingsSelect
"High quality (free unlimited storage)" and click the "Recover Storage"
button. Note that you should recover storage only when your free
storage gets full and not after every upload.

Photos and videos uploaded to folders will consume storage regardless of the size of the files.

Pictures
that are already present in your Google account will keep using the
same storage as was the case at the moment of uploading.

If
you think it is important that Picasa3 is updated a.s.a.p. (before
March 15, 2016) to support the "High quality" 16 MPixel free upload,
report this using Google Photos feedback: Link

Google gives you 15 GB free storage and it is shared across Drive, GMail, and Google Photos. If you'd like to check your Google storage (total storage used and available), go to the following link and hover over the pie chart: https://www.google.com/settings/storage There you can also change your storage plan and purchase more storage or downgrade to free storage.